readily if heated incautiously, and its surface will not keep good for any length of time. Owing to decomposition under the action of light, a layer of sulphuric acid forms on the surface, after which it is very difficult to restore the electrical virtue so remarkable in the new material, although washing with hot water or immersion in a blast of steam is said to be effective in some degree. The rubbers consist of two rectangular pieces of wood, hinged to supports attached to the framework of the machine, and fitted with springs and screws, so that they can be made to clip the plate with any required pressure. The rubbing surfaces are the other cushion, and so on. The electricity goes on accumulating in the prime conductor until the potential is so great that discharge by surface conduction, or by spark, takes place between the collectors and the cushion, or between the collectors and the axis. If it is desired to obtain negative electricity from a machine with a glass disc, we have simply to connect the prime conductor to earth, insulate the cushions, and collect the electricity from them. We have said that there is a limit to the potential to which the charge on the prime conductor can be raised. We can never get a longer spark from the machine than the length of the interval between the collector and the cushion or the axis, as the case may be. The limiting potential can, however, be increased by insulating the axis of the machine, or making the axis itself wholly or partially of insulating material, and by using only one rubber and one collector, and placing them at the extremities of a diameter. The machine of Le Roy, often called Winter's machine (fig. 57), is constructed on this pattern. We get, of course, cæteris paribus, only half as much electricity per revolution with a machine of this kind as with Ramsden's; but the spark is longer, in consequence of the greater insula tion between the cushion (A) and the collector (B). FIG. 56.-Ramsden's electrical machine. usually formed of leather, stretched as smooth and flat as possible (oiled silk is sometimes used, but it is not so durable). Before the leather cushions are fit for use, they must be carefully coated with amalgam. Th: amalgam most commonly used is Kienmayer's, which is a composition of two parts of mercury with one of zinc and one of tin. A great variety of different compounds of this kind have been used by different electricians, bisulphide of tin being a general favorite. The amalgam must be powdered as finely as possible, all grit being carefully removed. The cushions are then to be lightly smeared with lard, and worked together till the surface is very smooth and the greasiness almost gone; then the amalgam is to be carefully spread over them, and the surfaces again worked together till a uniform metallic surface is attained; they are then ready for use. The amalgam aids the action of the machine in two ways,-first, by presenting a surface which is highly negative to glass; secondly, by allowing the negative electricity evolved by friction to flow away without hinderance from the points of contact. In order to secure the second of these advantages still more perfectly, the cushions should be carefully connected by strips of tinfoil, or otherwise, with knobs, which can be put to earth during the action of the machine. The collectors are two stout metal forks bestriding the glass disc at the ends of a horizontal diameter. They are armed, on the sides next the glass, with rows of sharp points, which extend across the rubbed part of the disc. The prime conductor in the specimen we are describing forms a metal arch rising over the framework of the machine, and insulated from the sole by two glass pillars. Various forms are given to this part of the machine, according to the fancy or convenience of the experimenter. One important thing to be seen to is, that there be no salient points on it which might facilitate the dissipation of electricity by brush, convective, or spark discharge. After what has been said, the action of the machine requires little explanation. The disc, electrified positively by contact with the amalgam, carries away a positive charge, whose potential rises rapidly as it leaves the cushion,-so high, in fact, that there is a tendency to discharge to the air, which is prevented by covering the excited parts of the disc by pieces of oiled silk. When the highly charged glass comes opposite the points of the collector, owing to the inductive action, negative electricity issues from the points and neutralizes the charged plate, which at this point is virtually inside a closed conductor. The result of this is that the prime conductor becomes positively charged. The neutralized parts of the disc now pass on to be rubbed by 1 Mascart, l. c. FIG. 57. Le Roy's machine. The cylinder machine, also called Nairne's macnine, was one of the first machines in which all Nairne's the essential parts of the modern frictional machine machine. appeared. It consists of a glass cylinder, which can be turned about a horizontal axis by a multiplying gear, or (as is now more usual) by means of a winch handle simply. The cushion is affixed to one horizontal metal cylinder, and the collector to another. It is necessary to insulate the axis in this machine, owing to its proximity to the ends of the conductors. Positive or negative electricity can be obtained with equal readiness by insulating either of the conductors, and connecting the other with the earth. Those who desire more minute information concerning the functions of the different organs of the frictional machine, are referred to Mascart, tom. ii. 834, etc. In the same place will be found a description of the famous machine with double plates constructed by Cuthbertson for Van Marum, and still to be seen in Teyler's Museum at Haarlem. A description of another of Van Marum's machines will be found in the article "Electricity" in the Encyclopædia Metropolitana. We take this opportunity of calling the scientific reader's attention to that article, which contains a great quantity of very valuable matter. Much of the work of the earlier electricians that we have been obliged to pass over in silence is fully described there. Electric machines have also been constructed of less costly materials than glass or even vulcanite-of cloth and paper, for instance-for an account of these, see Riess, Bd. ii. 2936, 937. Many experiments have been made on the electrification of sifted powders. We have already, in describing Lichtenberg's figures, alluded to some cases of this kind. As a rule, either the results are Friction of powders, etc. very uncertain, or the conditions of the experiment very complicated, so that the experiments are, in most cases, more curious than valuable, from a scientific point of view. Such as desire it will find abundant indications of the sources of information in Riess, Bd. i. 27 938 #qq., and Ency. Metrop., art. Electricity," 193 eqq. One case of this kind, however, was so famous in its day, that we ought to mention it. In the year 1840 a workman at Newcastle, having accidentally put one hand in the steam which was blowing off at the safety valve of a high-pressure engine boiler, while his other hand was on the lever of the valve, experienced a powerful electric shock in his arms. Armstrong investigated the matter, and was led to construct his famous hydro-electric machine. This apparatus consists simply of an insulated boiler for generating highpressure steam, fitted with a series of nozzles, kept cool by a stream of water. The steam issues from these nozzles and impinges on a conductor armed with points for collecting the electricity. The boiler gets electrified to a high potential, and a torrent of dense sparks may be drawn from it. The machine far surpassed any ordinary electrical machine in the quantity of electricity furnished in a given time. By means of it water was decomposed, and the gases collected separately. It was reserved for Faraday to trace the exact source of the electromotive force. He demonstrated, by a series of ingenious experiments, that the electrical action arose from the friction of the particles of water in the condensed steam against the wood of the nozzles.1 Electrophorus. If each time we charged the collectcr it were discharged by contact with the interior surface of a hollow conductor A, it is obvious that we could raise by a sufficient number of such contacts to as high a potential as we please, provided it were sufficiently well insulated. This remark brings Volta's electrophorus into the present category of electrical machines. In the rest of the induction machines to be described the excited di-electric is dispensed with, and an electrified conductor substituted in its place. The earliest apparatus that involved the principle of such machines appears to have been Ben- Bennet's net's doubler. The principle of this apparatus doubler. may be explained thus. Let A and C be two fixed discs, and B a disc which can be brought at will within a very short distance of either A or C. Let us suppose all the plates to be equal, and let the capacities of A and C in presence of B be each equal to p, and the coefficient of induction between A and B, or C and B, be q. Let us also suppose that the plates A and C are so distant from each other that there is no mutual influence, and that p' is the capacity of one of the discs when it stands alone. A small charge Q is communicated to A, and A is insulated, and B, uninsulated, is brought up to it; the charge on B will be-2Q. B is now uninsulated and brought to face C, which is uninsulated; the charge on C will be 2Q. Q. C is now insulated and connected with A, which is always insulated. B is then brought to face A and uninsulated, so that the charge on A becomes rQ, where A is now disconnected from C, and here the first operation ends. It is obvious that at the end of n such operations the charge on A will be "Q, so that the charge goes on increasing in geometrical progression. If the distance between the discs could be made infinitely small each time, then the multiplier r would be 2, and the charge would be doubled each time. Hence the name of the apparatus. Darwin, Cavallo, Darwin, Cavallo, and Nicholson' devised mechanism for effecting the movements which in Bennet's instrument were made by hand. Cavallo's Nicholson. was a reciprocating movement, but in the machines of Darwin and Nicholson the motion was continuous and rotatory. Nicholson's doubler is a very elegant instrument. A drawing of it is given by Mascart (t. ii. 845); the specimen there represented is very like one which was found among the late Professor Willis's apparatus, and is now in the Cavendish Laboratory at Cambridge. A still more elegant machine is "Nicholson's spinning condenser," which bears a remarkable resemblance to the induction machine of Töpler. A description, with a figure, will be found in the Encyclopædia Metroapolitana, art. "Electricity," 112. Remaining Cases. Of these the most important Miscellane are atmospheric electricity, which belongs properly ous results. to meteorology, animal electricity, comprehending the study of the properties of the electrical fishes, and the electric phenomena of nerve and muscle. We have already indicated the literature of the former subject, and the latter belongs, for the present at least, to physiology. Evaporation, combustion, and in fact chemical action generally, have been brought forward by some experimenters as sources of electro-motive force. About the last of all there is, of course, in one well-known case no doubt. As to the experiments generally alluded to under the other two heads-in particular those of Laplace and Lavoisier, Volta, Pouillet, and others-there has been considerable difference of opinion, and we need not occupy space here with fruitless discussion of the matter. Similar remarks apply to the electrification caused by pressure, cleavage, and rupture. Machines founded on Induction and Convection. The oldest electric machine on this principle is the electrophorus of Volta, 1775. This consists of a plate of resinous matter (now usually vulcanite) backed by a plate of metal, and a loose metal plate, which we may call the collector, fitted with an insulating handle. The vulcanite is electrified by flapping it with a cat-skin, the collector is placed upon it, uninsulated for a moment by touching it with the finger, and then lifted by the insulating handle. The collector plate is then found to be charged (positively) to high potential, and sparks of some length may be drawn from it. The explanation of the action of the electrophorus is simple enough, if we keep clearly in view the experimental fact that the surface electrification of a non-conductor, like vulcanite, will not pass to a metal plate in contact with it under ordinary circumstances. If the surface density of the electrification be very great, discharge to the metal may no doubt take place; and if the collector be kept for a very long time in contact with the vulcanite, it is said that it may become negatively electrified. In the normal state, however, the negative electricity of the vulcanite remains upon it, and the thin layer of sir intervening between it and the collector forms the di-electric in a condenser of very great capacity, so that a quantity of electricity collects on the lower surface of the condenser very nearly equal to that on the vulcanite. The difference of potential between the plates is very small (just as in Volta's condensing electroscope, see above, p. 32). When the collector is raised it carries away the positive charge-the potential of which, owing to the decrease in the capacity of the collector, rises enormously. It is to be noticed that the potential of the charge on the vulcanite rises to a corresponding extent. This remark partly explains the remarkable fact that, when the collector is kept on the excited vulcanite, its electrification may be kept for a long time (for weeks under favorable circumstances), whereas it speedily dissipates if the vulcanite be left uncovered. According to Riess, the fact that a plate of metal laid on an excited piece of glass tends to preserve its electrification was discovered by Wilcke in 1762. It is obvious that if any conductor be connected with the part of any of these machines corresponding to the conductor A in the above description, and the potential of A be raised to any small positive or negative value, we can by means of the machine increase the charge, and therefore the potential, up to any required amount. We have, in fact, an electric machine which may be used for all the ordinary purposes. It was not with this view, however, that these pieces of apparatus were first invented, but rather for the purpose of demonstrating small electric differences. In this they were but too successful, for it was found that it was impossible to prevent them from indicating electric differences unavoidably arising within the apparatus itself. It was this difficulty no doubt that led to their being ultimately abandoned, and for a time forgotten, although they were once in high favor. Of late, however, they have been taken up as electro-motors with great success. convecto The type of all these machines is an arrangement of the following description. A conductor or Typical carrier C, or a series of carriers, is fastened upon inductive the circumference of an insulating disc. At the machine. ends of a diameter are two hollow conductors, A and B, embracing the disc on both sides, so that twice in the course of a revolution the carrier is virtually in the interior of a hollow conductor. Inside each conductor are two springs one of these is in metallic connection with the conductor, and may be called the receiving spring; the other, called the inductor spring, is insulated from the conductor, and is connected either to earth or with the corresponding spring belonging to the other conductor. Suppose A to be at a small positive potential, and B at zero potential; starting with C in connection with the inductor spring inside A, it becomes negatively electrified and carries away its charge; it next comes in contact with the receiving spring in B, and, being now part of the interior of a hollow conductor, it parts with the whole of its Phil. Trans., 1787. Phil. Trans., 1788. • Pogg. Ann., 1865. By connecting the conductor with the positive or negative pole of a small galvanic battery, for instance, charge to B; then it passes on and is charged positively at B's | inductor spring; then discharges to A at A's receiving spring; and so on. The positive and negative charges are each a little increased every revolution, and the difference of potentials accordingly augmented. This is the principle of Varley's nachine (1860), and of Thomson's mouse mill and replenisher (1867); it is virtually that of Bennet's doubler. Water Closely allied to these machines is Thomson's water-dropping potential equalizer. This consists dropping of an insulated reservoir of water, with a long machine. pipe, from the nozzle of which water is allowed to break in drops. It is obvious that if the potential of the reservoir be above that of the air surrounding the spot where the water breaks into drops, each drop will carry away with it a positive charge, and this will go on till the potentials are equalized. This device was introduced by Thomson in observations on atmospheric electricity. The burning match which he uses in conjunction with the portable electrometer acts in the same way. He has also constructed a water-dropping electric machine on a similar principle. Two streams of water break into drops inside two inductors connected with the internal armatures of two Leyden jars, A and B; the drops from each inductor fall into a receiver connected with the other inductor. A very small difference of potential between the jars starts or reverses the action of the apparatus; in fact, it will in general start of itself, and very soon sparks are seen passing between the different parts, and the drops are scattered in all directions by the strong electrical forces developed. The most remarkable, as well as the most useful, of all these machines is that of Holtz. Here the convection is effected by means of a disc of glass, which is mounted on a horizontal axis F (fig. 58), and can be made to rotate with considerable angular velocity by means of a multiplying gear, part of which is seen at X. Holtz's machine. Close behind this glass disc is fixed another vertical disc of glass, in which are cut two windows, B, B. On the side of the fixed disc next the rotating disc are pasted two sectors of paper, A, A, with short blunt points attached to them, which run out into the windows towards the rotating disc, without quite touching it. Two metal combs C are placed on the other side of the rotating disc (that nearest the reader), the teeth being put opposite the parts of A, A which lie towards the windows. The combs are fixed to metal shanks, which pass through a stout horizontal bar of ebonite. One of these shanks terminates in a couple of balls at E, and the other carries a sliding electrode D with a long ebonite handle. The framework which carries the horizontal ebonite bar and supports the fixed plates, etc., will be understood from the figure. The machine, as originally constructed by Holtz, contained only the parts we have described. Poggendorff doubled all the parts (except, of course, the electrodes D and E). The figure represents Ruhmkorff's modification of this construction. Behind the fixed disc there is another fixed disc, with windows and armatures like the first, and, beyond that, another movable disc mounted on the axis F. The combs are double, as will be seen from the figure. To start the machine, D and E are brought together, and one of the armatures (or one pair), say the right-hand one, is electrified in any manner, let us say positively, and the disc set in rotation. After a little time a hissing noise is heard, and the machine becomes sensibly harder to turn, as if the disc were moving through a resisting medium. 1 Jenkin, Elect. and Mag., cap. xix. Described in the art. ELECTROMETER. Pogg. Ann., 1865. If the room be dark, long curved pencils of blue light will now be seen issuing from the points of the left-hand comb, and running along the surface of the disc in a direction opposite to its motion, while little stars shine upon the points of the righthand comb. After this state has been reached, the balls D, E may be separated, and a continuous series of brush discharges will take place between them, even when the distance is very considerable. If two Leyden jars, L, L, be hung upon the conductors which support the combs, the outer coatings being connected by a conductor M, then a succession of brilliant and sonorous sparks will take the place of the brushes. Instead of using the two jars L, L, we may connect D and E with the internal and external armatures of a condenser; it will then be found that, as we augment the capacity of the condenser (the angular velocity of the disc being constant), the frequency of the sparks diminishes, while their brilliancy increases. If we insert a high resistance galvanometer between D and E, it will indicate a current flowing from D to E, the intensity of which, under given atmospheric conditions and given state of the machine, will vary as the angular velocity, being independent, within very wide limits, of the resistance between D and E. It is not difficult to give a general account of the action of this machine, although it is very hard to assign the precise importance of the individual parts, very slight modifications of which greatly affect the efficiency. Suppose D and E in contact; the right-hand armature, charged +, acts by induction on the right-hand comb, causing - electricity to issue from the points upon the disc. At the same time the positive electricity of the right comb passes through DE to the left comb, and issues from its teeth upon the parts of the disc at the other end of the horizontal diameter. This + electricity eler trifies the left armature by induction, + electricity issuing from the blunt point upon the further side of the rotating disc. The charges thus deposited on the disc are carried along, so that the upper half is electrified on both sides, and the lower half + on both sides, the sign of the electrification being reversed as the disc passes between the combs and the armature by the electricity issuing from the combs and from the armatures. If it were not for dissipation in various ways, the electrification everywhere would obviously go on increasing; but in practice a stationary condition is soon attained, in which the loss from the armatures is just balanced by the gain owing to the action of the blunt points. After this, both sides of the disc are similarly electrified, the upper half always, the lower always ++ electricity continually issuing from the points of the right comb, electricity from the points of the left. This is, of course, accompanied by a current of + electricity from right to left through DE. tion of Holtz's The machine of Holtz, as we have described it, is somewhat uncertain in its action in our moist Modifica climate; but a slight modification of it gives excellent results. Upon the axis X is fixed a disc of machine." ebonite, large enough just to reach between the paper armatures. This disc is fitted with a small rubber attached to the frame of the apparatus, and forms a small electric machine, which keeps the armatures continually electrified. The whole is inclosed in a glass case, with a beaker of sulphuric acid to dry the air. There is a machine of this kind at present in the Cavendish laboratory at Cambridge, which never fails when the auxiliary apparatus is at all in good order. A very remarkable phenomenon often occurs when the electrodes of Holtz's machine are in connection with the armatures of a condenser of considerable capacity, and are so far separated that a spark does not pass. The machine charges the condenser up to a certain point, and then the condenser discharges along the surface of the disc. If the experiment be conducted in a dark room, a flash of light will be seen to pass along the surface of the disc, and thereafter it will be observed that the long positive brushes have shifted from one combo the other; after a little the condenser discharges again, and the brushes will now be seen in their old place, and so on. This phenomenon, though interesting to study, is often inconvenient in practice. To prevent it, Holtz introduced the diagonal conductor which is seen on many machines. For an account of this, and for other details concerning these machines, we refer the reader to Mascart, t. ii. 847 sqq., whose account of the more obvious principles of this apparatus is among the most lucid we have seen. His account of the experiment of causing one Holtz's machine in action to turn the disc of another by the electrical reaction is of peculiar interest. Induction Electro-magnetic Induction Machines.-The type of these is the induction coil or inductorium, some- oil. times called Ruhmkorff's coil, after the great Parisian instrument-maker who first brought the instrument to perfection. The object of such machines is to obtain great eleetro-motive force from sources which furnish large quantities of electricity, but have only small electro-motive force. 4 We speak of resistances of 1 to 10,000 or 100,000 ohms. The line of division is not horizontal, however, if, indeed, it be exactly a diameter. See Mascart. Compare Carré's machine, Mascart, t. ii. 856. The principles on which the action is founded have been sufficiently indicated above in our section on the induction of electric currents. We have also given in the Historical Sketch (pp. 12, 13) some notices of the literature of the subject; a brief enumeration of the essential parts of the machine is all that is necessary here. We have first the primary coil-of thick wire and few windings, so as to have a small resistance and a small coefficient of self-induction; the secondary coil surrounding the primary is of thin wire (mm. or so), with many windings, the length in large machines being often 100,000 metres. In order to avoid the danger of disruptive discharge between parts of the insulated wire, the coil is divided up by insulating septa, so that parts at very different potentials are separated. In the centre of the primary is placed a bundle of iron wires; this greatly strengthens the action, and a good deal depends on the quality of the iron, which should be very soft. The interruptor is simply a lever, worked by the coil itself or by an electro-magnet separate from the coil, by means of which the circuit of the primary is made and broken automatically. A variety of forms have been given to the part of the apparatus; the interruptor of Foucault is a very common one.1 For some purposes a break driven by clock-work is used. The condenser, a very important part of the apparatus, is made of a number of sheets of tinfoil, interleaved with sheets of oiled silk or varnished paper. One set of leaves of the condenser is connected with one side of the break, and the alternate set with the other side. The function of the condenser is to provide a way for the electricity when the circuit is broken, and thus to prevent the intense spark of the extra current in the primary, which destroys the contact surfaces of the break, and, what is worse, prolongs the fall of the primary current, and thereby reduces the average electro-motive force of the induction current. Other devices have been tried for effecting the same object as the condenser, such as inserting a fine metallic wire or an electrolyte as an alternative circuit to the break; and these answer the purpose to a considerable extent. An important improvement affecting this part of the apparatus has recently been introduced by breaking the primary circuit between the poles of a magnet, the effect of which is that the spark is suddenly drawn aside (blown out as it were). A considerable increase of striking distance between the poles of the secondary results from this arrangement. ABSOLUTE MEASUREMENTS. We have already indicated the considerations which determine the fundamental units in the two systems that have come into practical use. We ought now to explain how practical standards can be constructed to represent these fundamental units, or at least known multiples of them. It is necessary to have such standards in order that we may be able to measure electrical quantities in absolute measure by simple and expeditious methods of comparison, it being obviously impossible in practice to make absolute measurements directly on all occasions. Electro-statical System.-By means of Thomson's absolute electrometer we can determine any electro-motive force in absolute measure. E. M. F. In this way Thomson found the electro-motive force of Daniell's battery to be 00374 C.G.S. electro-statical units. Resistance. Measure of subject; such an attempt would lead us into technical partioulars intelligible only to a few scientific men. We are fortunate, however, in being able to refer the English reader to two books which contain in a collected form all, or nearly all, the requisite information, viz., Maxwell's Electricity and Magnetism, and the collected Reports of the Committee of the British Association on Electrical Standards. As a specimen of the theoretical considerations involved, the reader may take Maxwell's method Resistfor determining the coefficient of self-induction of ance. a coil (given above, p. 75). If we know the value of L (in centimetres) from calculation, then equation (33) might be used to find a in absolute measure. This would not be a practicable method, inasmuch as the calculation of L would be difficult if not impossible; we might, however, determine L by comparison with a coefficient of mutual induction which could be calculated. Kirchhoff. The earliest absolute measurement of the resistance of a wire (by Kirchhoff in 1849) was of the kind just alluded to; that is to say, it involved the comparison of a resistance with a coefficient of mutual induction, the time measurement being that of the period of oscillation of a galvanometer. Weber used two methods,-(1) the method of transient currents, in which he measured the throw Weber. of a galvanometer caused by the current from an earth inductor of known area when it was turned about a vertical axis, so that the number of the earth's lines of force through it increased from zero to a maximum; and (2) the method of logarithmic decrements, in which he observed the time of oscillation and the logarithmic decrement of a magnet in a galvanometer of known constant. In the last of these two methods the horizontal component of the earth's horizontal force comes in directly, and the magnetic moment of the galvanometer magnet must be determined, which is a matter of great difficulty. The determination of the British Association committee was carried out by Messrs. Maxwell, B. A. committee. Balfour Stewart, and Fleeming Jenkin, and the result of it was the construction of a standard called the ohm, which professes to represent a velocity of an earth quadrant per second (109 -).-The method they used is due to Sir Wm. Thomson. It consists essentially in causing a coil of wire of known dimensions to rotate about a vertical axis, and observing the deflection of a magnet of very small moment suspended at its centre. cm. sec. rausch. In a recent determination, F. Kohlrausch has of the coil called the ohm is equal to 1.0493 mercury units. By using the absolute electrometer (see art. ELECTROMETER), or another that had been compared with Lorenz has, still more recently, made a deterit, we could by the method given above, p. 44, find mination of the value of the mercury unit in abso- Lorenz. a resistance (which was large enough to suit the method) in lute measure. He causes a copper diso to rotate inside a coil electro-statical measure. of known dimensions. The two ends of a circuit C are kept in Then, having standards of electro-motive force contact with the axis and circumference respectively of this Current. and resistance, we could easily measure a current disc. At two points A and B of C, the resistance between in electro-static measure by applying Ohm's law. The same which is R, are attached the two terminals of the coil of wire, thing might be done by constructing the standard of quantity, in circuit with which is also a battery. A sensitive galvanomwhich is the charge on an isolated sphere of unit radius charged eter is placed in the circuit C, and the angular velocity of the to unit potential. By comparing the throw of a galvanometer disc is adjusted till this galvanometer indicates no current. If when unit quantity is discharged through it with the deflection n be the number of revolutions per second, and E the electroproduced by any current, we could determine the latter in abso- motive force of induction per unit of inducing current, caloulute measure by observing the time of oscillation of the galva-lated from the dimensions of the coil, then the resistance R is nometer and the logarithmic decrement of its oscillation (see equal to nE in electro-magnetic measure. Maxwell, vol. ii. § 749). The result obtained by Lorenz for the value of the mercury unit is ⚫9337 Earth quadrant ; this would make the value of Second Earth quadrant the B. A. standard ⚫9797 Among the absolute measurements in the present Di-electric system of units, we must not omit to mention Sir strength. Wm. Thomson's determinations of the di-electrio strength of different thicknesses of air. From these, and from the measurement of the electro-motive force of Daniell's cell just mentioned, he concluded that a Daniell's battery of 5510 elements would be competent to produce a spark between two slightly curved metallic surfaces at of a centimetre asunder in ordinary atmospheric air.3 Electromagnetic measure. Electro-magnetic System.-The great majority of the absolute determinations hitherto made have reference to this system. We make no attempt here to instruct the reader concerning the details of this 1 See Wiedemann's Galv., or Du Moncel, Notice sur l'Appareil de Ruhmkorff. Reprint of Papers, ? 305, etc. Reprint of Papers, § 340. Second There is thus considerable discordance between the different results. It is a curious fact that the mean of the result of 4 Such as wish to go deeply into the matter must read the Maas 8 Since the above was written, an account has appeared of a new [Son (1833-85) of a naval captain, graduated at Genoa, engineer for first and other Atlantic cables, determined electric measures, professor in London and Edinburgh, built telpher line in Tunis.-AM. ED.] VOL. VIIL-334 Calorimetric method. Besides these methods, there is yet another of a totally different character, originally suggested by Thomson in 1851, in his paper on the "Mechanical Theory of Electrolysis." This method consists in measuring the amount of heat developed in a wire by a current the square of whose strength is known in electro-magnetic measure. If we know the mechanical equivalent of heat with sufficient accuracy, we can calculate from these results the resistance of the wire in absolute measure by means of Joule's law. Measurements of this nature have been made by Von Quintus Icilius, Joule, and H. Weber.3 We can, by means of a tangent galvanometer, Current. find the value of any current in electro-magnetic measure (see art. GALVANOMETER). If the resistance of the circuit be found, by comparison with the ohm or other absolute standard, we can determine the value of the electro-motive force in the circuit by Ohm's law. Measurements of this kind have been made by Bosscha, by Von Waltenhofen, F. Kohlrausch, and Latimer Clark. The results of Kohlrausch for the cells of Daniell and Grove, when no current is passing, are 1138 × 105 and 1942 x 105 C.G.S. units respectively. Latimer Clark gives 1110 × 105 and 1970 x 105 for the same constants. The results, of course, depend on the constitution of the cells. Taking the number of electro-magnetic units in an electrostatic unit to be 3 X 1010, we get from Thomson's electro-static measurements for the electro-motive force of Daniell's element 1120 105 in C.G.S. units.7 The agreement among the different results is so far good. The determination of the electro-chemical equivalent of some elementary substance in this system of units is of great importance. Determinations exist by Weber, Bunsen, Casselmann, Joule, and F. Kohlrausch. The result of the last is no doubt the best, as he combined with his voltametric experiments a determination of the horizontal component of earth's magnetic force, which is the most uncertain factor in the result. According to his result, one C.G.S. unit of electricity deposits 011363 (±·000002) gm. of silver. From this we get for the electro-chemical equivalent of water 0009476. Electrochemical equivalent. Measure etc. cm. 800. re THEORIES OF ELECTRICAL PHENOMENA. Throughout this article we have limited ourselves as much as possible to an exposition of the Speculative theories, experimental facts of electricity. Where mathematical developments have occurred, they have in most cases been simply deductions from some principle or principles well established by experience. To have made our survey of the present state of electrical science complete, we ought to have added a section on the different attempts which have been made by the doctors of the science to penetrate a little farther into the secrets of the hidden mechanism by which electrical phenomena are brought about. But any attempt at a review of this kind must be relinquished. We refer the reader to our indications of the literature (Historical Sketch, p. 11). The most important work in this department lies at hand for the English reader in Professor Clerk Maxwell's Treatise on Electricity and Magnetism. Particularly important are his theory of electric displacement and its application to statical as well as to current electricity; his investigation of the stresses in the medium, by which the electrostatical forces on the one hand, and the electro-magnetic forces on the other, may be produced; the application of the theory of displacement to the case of electrical equilibrium when the di-electric medium is not everywhere the same; the dynamical theory of the electro-magnetic field; and the electro-magnetic theory of light. Maxwell gives, at the end of his work, a most instructive summary of the different speculative theories. The student who desires to pursue this department farther will do well to master this summary at the outset. (G. CH.) INDEX. Ratio of Electro-static to Electro-magnetic Unit.ment of the If we measure the same quantity of electricity first funda- in electro-static and then in electro-magnetic measmental veure, the fundamental units of mass, length, and locity. time being the same in both cases, the ratio of the two measures will vary directly as the magnitude of the unit of length, and inversely as the magnitude of the unit of time adopted. This ratio may therefore be regarded as a velocity Absolute measurements, 97, 98; history of Gauss, Weber, B. A. Committee, etc., 15. Accumulator, theory of, 32. Alternating discharges with Inductorium and Leyden jar, 61. Ampère, electro-dynamics, 10. tal arrangements for show- Batteries, 87, 88; history of, 13; one-fluid and two-fluid -oxidizing agents in-local action-polarization, etc., 87. Battery of Leyden jars, 33. Capacity, coefficients of, 26. Cell of Daniell, different mod The figures refer to the pages. ifications of, 87, 88; of | Convection, electrolytic, Helm- Circular current, magnetic Contact force, general law of, Contact of conductor with Contact of non-conductors, 93. cm. the Siemens unit. This would make the B. A. unit 1-0014 X 109 holtz, 82; of heat, electric, Decomposition of alkalis, Davy, solids, 62; in gases, magnetic action on, De la Rive and Plücker, 70. Disruptive discharge, 56-63; theoretical considerations on, 58; progress of, 58. Distribution, electrical, Coulomb, 19, 21-23; general problem of, 25. Doubler, Bennet's, Darwin, Cavallo, and Nicholson, 95, 96. Earth's action on suspended Electro-dynamometer, use of, Maxwell, Elect. and Mag., vol. ii. § 768. • We have followed throughout the views expounded in this work; and we are also under great obligations to its author for his advice on many points. For aid in collecting facts we are indebted mainly to the works of Riess, Wiedemann, and Mascart. Without their aid many sections of this article could not have been written. Wiedemann's treatise, in particular, lightened our task by the extent of its information and the profusion and accuracy of its references to original authorities for the facts in electrical science. |